Cancer afflicts approximately 1.2 million people in the United States each year. About 50% of these cancers are curable with surgery, radiation therapy, and chemotherapy. Despite significant technical advances in these three types of treatments, each year more than 500,000 people will die of cancer in the United States alone. (Jaffee, E. M., Ann. N.Y. Acad. Sci. 886:67-72 (1999)). Because most recurrences are at distant sites such as the liver, brain, bone, and lung, there is an urgent need for improved systemic therapies.
The goal of cancer treatment is to develop modalities that specifically target tumor cells, thereby avoiding unnecessary side effects to normal tissue. Immunotherapy has the potential to provide an alternative systemic treatment for most types of cancer. The advantage of immunotherapy over radiation and chemotherapy is that it can act specifically against the tumor without causing normal tissue damage. One form of immunotherapy, vaccines, is particularly attractive because they can also provide for active immunization, which allows for amplification of the immune response. In addition, vaccines can generate a memory immune response.
The possibility that altered features of a tumor cell are recognized by the immune system as non-self and may induce protective immunity is the basis for attempts to develop cancer vaccines. Whether or not this is a viable strategy depends on how the features of a transformed cell are altered. Appreciation of the central role of mutation in tumor transformation gave rise to the hypothesis that tumor antigens arise as a result of random mutation in genetically unstable cells. Although random mutations might prove immunogenic, it would be predicted that these would induce specific immunity unique for each tumor. This would be unfavorable for development of broadly effective tumor vaccines. An alternate hypothesis, however, is that a tumor antigen may arise as a result of systematic and reproducible tissue specific gene deregulation that is associated with the transformation process. This could give rise to qualitatively or quantitatively different expression of shared antigens in certain types of tumors that might be suitable targets for immunotherapy. Early results, demonstrating that the immunogenicity of some experimental tumors could be traced to random mutations (De Plaen, et al., Proc. Natl. Acad. Sci. USA 85: 2274-2278 (1988); Srivastava, & Old, Immunol. Today 9:78 (1989)), clearly supported the first hypothesis. There is, however, no a priori reason why random mutation and systematic gene deregulation could not both give rise to new immunogenic expression in tumors. Indeed, more recent studies in both experimental tumors (Sahasrabudhe et al., J. Immunol. 151:6202-6310 (1993); Torigoe et al., J. Immunol. 147:3251 (1991)) and human melanoma (van Der Bruggen et al., Science 254:1643-1647 (1991); Brichard et al., J. Exp. Med. 178:489-495 (1993); Kawakami et al., Proc. Natl. Acad. Sci. USA 91:3515-3519 (1994); Boel et al., Immunity 2:167-175 (1995); Van den Eynde et al., J. Exp. Med. 182: 689-698 (1995)) have clearly demonstrated expression of shared tumor antigens encoded by deregulated normal genes. The identification of MAGE-1 and other antigens common to different human melanoma holds great promise for the future development of multiple tumor vaccines.
In spite of the progress in melanoma, very few shared antigens recognized by cytotoxic T cells have not been described for other human tumors. The major challenge is technological. The most widespread and to date most successful approach to identify immunogenic molecules uniquely expressed in tumor cells is to screen a cDNA library with tumor-specific CTLs (cytotoxic T lymphocytes). Application of this strategy has led to identification of several gene families expressed predominantly in human melanoma. Two major limitations of this approach, however, are that (1) screening requires labor intensive transfection of numerous small pools of recombinant DNA into separate target populations, which themselves often need to be modified to express one or more MHC molecules required for antigen presentation, in order to assay T cell stimulation by a minor component of some pool; and (2) with the possible exception of renal cell carcinoma, tumor-specific CTLs have been very difficult to isolate from either tumor infiltrating lymphocytes (TIL) or PBL of patients with other types of tumors, especially the epithelial cell carcinomas that comprise greater than 80% of human tumors. It appears that there may be tissue specific properties that result in tumor-specific CTLs being sequestered in melanoma.
Direct immunization with tumor-specific gene products may be essential to elicit an immune response against some shared tumor antigens. It has been argued that, if a tumor expressed strong antigens, it should have been eradicated prior to clinical manifestation. Perhaps then, tumors express only weak antigens. Immunologists have long been interested in the issue of what makes an antigen weak or strong. There have been two major hypotheses. Weak antigens may be poorly processed and fail to be presented effectively to T cells. Alternatively, the number of T cells in the organism with appropriate specificity might be inadequate for a vigorous response (a so-called “hole in the repertoire”). Elucidation of the complex cellular process whereby antigenic peptides associate with MHC molecules for transport to the cell surface and presentation to T cells has been one of the triumphs of modern immunology. These experiments have clearly established that failure of presentation due to processing defects or competition from other peptides could render a particular peptide less immunogenic. In contrast, it has, for technical reasons, been more difficult to establish that the frequency of clonal representation in the T cell repertoire is an important mechanism of low responsiveness. Recent studies demonstrating that the relationship between immunodominant and cryptic peptides of a protein antigen change in T cell receptor transgenic mice suggest, however, that the relative frequency of peptide-specific T cells can, indeed, be a determining factor in whether a particular peptide is cryptic or dominant in a T cell response. This has encouraging implications for development of vaccines. With present day methods, it would be a complex and difficult undertaking to modify the way in which antigenic peptides of a tumor are processed and presented to T cells. The relative frequency of a specific T cell population can, however, be directly and effectively increased by prior vaccination. This could, therefore, be the key manipulation required to render an otherwise cryptic response immunoprotective. These considerations of cryptic or sub-dominant antigens have special relevance in relation to possible immune evasion by tumors through tolerance induction. Evidence has been presented to suggest that tumor-specific T cells in the tumor-bearing host are anergic, possibly as a result of antigen presentation on non-professional APC (Morgan, D. J. et al., J. Immunol. 163:723-27 (1999); Sotomayor, E. M. et al., Proc. Natl. Acad. Sci. U.S.A. 96:11476-81 (1999); Lee, P. P. et al., Nature Medicine 5:677-85 (1999)). Prior tolerization of T cells specific for immunodominant antigens of a tumor may, therefore, account for the difficulty in developing successful strategies for immunotherapy of cancer. These observations suggest that T cells specific for immunodominant tumor antigens are less likely to be effective for immunotherapy of established tumors because they are most likely to have been tolerized. It may, therefore, be that T cells specific for sub-dominant antigens or T cells that are initially present at a lower frequency would prove more effective because they have escaped the tolerizing influence of a growing tumor.
Another major concern for the development of broadly effective human vaccines is the extreme polymorphism of HLA class I molecules. Class I MHC:cellular peptide complexes are the target antigens for specific CD8+ CTLs. The cellular peptides, derived by degradation of endogenously synthesized proteins, are translocated into a pre-Golgi compartment where they bind to class I MHC molecules for transport to the cell surface. The CD8 molecule contributes to the avidity of the interaction between T cell and target by binding to the α3 domain of the class I heavy chain. Since all endogenous proteins turn over, peptides derived from any cytoplasmic or nuclear protein may bind to an MHC molecule and be transported for presentation at the cell surface. This allows T cells to survey a much larger representation of cellular proteins than antibodies which are restricted to recognize conformational determinants of only those proteins that are either secreted or integrated at the cell membrane.
The T cell receptor antigen binding site interacts with determinants of both the peptide and the surrounding MHC. T cell specificity must, therefore, be defined in terms of an MHC:peptide complex. The specificity of peptide binding to MHC molecules is very broad and of relatively low affinity in comparison to the antigen binding sites of specific antibodies. Class I-bound peptides are generally 8-10 residues in length and accommodate amino acid side chains of restricted diversity at certain key positions that match pockets in the MHC peptide binding site. These key features of peptides that bind to a particular MHC molecule constitute a peptide binding motif.
Hence, there exists a need for methods to facilitate the induction and isolation of T cells specific for human tumors, cancers and infected cells and for methods to efficiently select the genes that encode the major target antigens recognized by these T cells in the proper MHC-context.